PlantUML is an open-source tool allowing users to create UML diagrams from a plain text language. Developed by Arnaud Roques
License: Open Source
OmniGraffle is a feature-rich diagramming tool with a strong emphasis on visual aesthetics and a user-friendly interface, ideal for creating detailed diagrams. In contrast, PlantUML is a lightweight, text-based UML diagram tool better suited for technical users who prefer version control and collaboration in code-based environments.